The present invention relates to suppression of noise in electromagnetic surveying and, more particularly, to a method and system for suppression of swell-induced electromagnetic noise that includes stacking two or more frames of electromagnetic data.
One type of marine geophysical surveying includes electromagnetic (“EM”) surveying, in which geophysical data may be collected from below the Earth's surface. Marine EM surveying is used, among other purposes, to infer spatial distribution of electrical conductivity of rock formations below the bottom of a body of water, such as a lake or ocean. The spatial distribution of conductivity is used to assist in determining the presence of hydrocarbon-bearing rock formations in the subsurface, potentially resulting in cost saving by better targeted drilling operations. One type of such surveying is known as “controlled source” electromagnetic (“CSEM”) surveying, which generally includes inducing a time-varying EM field in the subsurface formations and measuring one or more parameters related to a response of the subsurface rock formations to the induced EM field. More particularly, CSEM surveying may use a controlled source that sends current between two electrodes. The current may diffuse through the sea water and subsurface formation with an electric potential difference caused by the current measured at some distance away from the source. The sensitivity of the potential difference to resistive anomalies in the subsurface formation, for example, a hydrocarbon reservoir, depends on the sensor offset and the source signal frequency. The ability to detect resistive anomalies at depth may be limited by a combination, of sensitivity to the anomalies and the signal-to-noise ratio.
One source of noise that may decrease the signal-to-noise ratio and, thus, limit the ability to detect resistive anomalies is swell. Swell is a highly synchronized wave motion of possibly high amplitude. Swell waves in a fully developed sea are minimally affected by local wind, but rather have outrun the storm that created them, lengthening and reducing in height in the process. Swells typically organize into groups smooth and regular in appearance. They are able to travel thousands of miles unchanged in height and period. At shallow and intermediate depths at sea, swell may be a major source of noise in EM surveying, such as CSEM surveying. The swell-induced noise may be particularly pronounced in water depths less than a few hundred meters. Swell typically may create noise in the approximate frequency range of 0.05 Hz to 0.1 Hz, which may coincide with the typical frequency range of optimal sensitivity to deep anomalies. Accordingly, data quality of the EM survey may be negatively impaired where significant swell is present. As result, EM surveys may be impracticable in some instances, thus limiting the weather window for EM surveys. For example, swell-induced noise may be a limiting factor on the height of the swells in which the survey vessel can operate.
Accordingly, there is a need for improved methods and systems for suppressing, swell-induced noise in EM surveying, for example, to improve data quality and enlarge the weather window for EM surveys.